1. Field of the Invention
This invention relates to apparatus for spraying liquid chemical onto the outer surface of a string of pipe or rod moving into or out of a wellbore.
2. Description of the Prior Art
Corrosion and hydrogen embrittlement have long been factors which have deleteriously affected pipe and rod strings. There are known liquid chemicals which may be applied to a string to alleviate this deterioration. However, it frequently happens that it is necessary to apply such liquid when the string is being tripped in or out of the wellbore.
For this purpose, an applicator system is required. Since the chemicals involved are expensive, such an applicator system should be adapted to minimize chemical loss.
Heretofore, John Grant has disclosed, in U.S. Pat. Nos. 3,306,310, 3,334,639, 3,378,088 and 3,475,781, an applicator system which involves the following features. It comprises an applicator which has an annular housing. This housing is mounted on the wellhead or is bolted to the I-beams of the rig sub-structure. In other words, the housing is fixed. Mounted within the housing is a ring of spaced nozzles; the nozzles are adapted to spray the string passing vertically through the central opening of the annular applicator. The nozzles are supplied by suitable means with pressurized liquid chemical.
Because the Grant applicator is fixed, the central opening through which the string extends has to be larger than the outside diameter of the string. To quote the patents, there has to be "ample clearance" between the housing and the string. This is so because the pipe or rod string has a tendency to swing back and forth laterally as it is being pulled from or run into the wellbore. If the applicator housing is contacted by the swaying string, it will likely be damaged.
Grant's system therefore involves the concept of a fixed housing with ample clearance, containing a fixed ring of nozzles, used in conjunction with a laterally moving string. There are disadvantages arising from this approach. Excess liquid chemical, which does not adhere to the string, is lost, as it drops through the clearance. Also, the "standoff" or distance between the nozzles and the string is constantly changing; this means that the pressure of the spray contacting the string varies, as does the spray pattern.
There is therefore still a need for an applicator system which is adapted to conserve or recover excess chemical and whose spray pattern and pressure at the string surface is generally constant.